Listeria monocytogenes is a food-borne human and animal pathogen that causes serious illness in pregnant women and immunocompromised individuals, including AIDS patients. A key determinant of virulence for this intracellular pathogen is its ability to exploit the host cell actin cytoskeleton for intracellular movement and intercellular spread. The long-term objective of this proposal is to determine how the L. monocytogenes surface protein ActA initiates the accumulation of actin filaments around the bacterium, channels the polymerization of these actin filaments into a productive, motile force, and determines the rate of filament elongation and consequently the rate of bacterial movement. To this end, I will first isolate missense alleles and dominant negative alleles of actA and analyze the resulting mutant proteins in vivo and in vitro for their ability to nucleate actin, generate directional movement, and interact with host proteins. Second, I will test the hypothesis that a critical concentration of ActA is needed for the bacteria to initiate movement within host cells by altering the levels of actA expression. And finally, because recent biochemical and genetic evidence indicate that ActA forms homodimers, I will isolate actA mutants that are specifically defective in dimerization. These mutants will be used to determine whether dimer formation is necessary for ActA function, and if so, for what aspect of ActA function.